By Bill Rigby

SEATTLE (Reuters) - Republican presidential candidate Mitt Romney on Thursday threatened trade sanctions against China if the world's No. 2 economy does not halt what he said was currency manipulation, unfair subsidies and rampant intellectual property theft.

"On day one of my administration I would designate China as a currency manipulator," Romney told to Microsoft Corp employees, in some of his toughest language yet against China.

"I would apply countervailing duties on Chinese goods where they have stolen intellectual property or where their currency manipulation is killing American businesses and jobs unfairly," he said at the software giant's headquarters in Redmond, Washington, near Seattle.

The attack on China was the most aggressive expression of the tough stance Romney set out last month, as he sought to stake out differences with President Barack Obama and tap into the U.S. public's rising concern over China's economic and military growth.

His speech, echoed remarks from the most recent Republican presidential candidate debate, in which he labeled China's policies as "cheating" and promised, if elected president, to immediately tackle China's currency policies with a complaint to the World Trade Organization.

Romney received a warm reception from the audience of about 300 Microsoft employees. The world's largest software maker is especially interested in intellectual property issues, having lost billions of dollars in Chinese sales over the years due to piracy.

The former Massachusetts governor is leading the field of Republicans seeking the nomination to run against Obama in 2012. But fewer than one in four, or 23 percent, of the party's voters, back him as rival Herman Cain gains ground, according to a Reuters/Ipsos poll released on Wednesday.

Romney believes the Obama administration has been weak on advancing overseas agreements that might boost free trade, although Wednesday's vote by Congress to approve long-delayed trade pacts with South Korea, Colombia and Panama may take the edge off such criticism.

On the currency issue, the U.S. Senate approved a controversial bill aimed at forcing China to raise the value of the yuan earlier this week, in an effort to save American jobs, but its fate in the House of Representatives is uncertain. China denounced the vote as a protectionist step.

Romney dismissed that legislation as "political theater," saying the President already had the powers he needs to tackle China on currency.

"We don't need new legislation, we need a new president," he told the audience at Microsoft.

Earlier on Thursday, Romney named three of former President George W. Bush's advisers to his campaign's trade team, including Carlos Gutierrez, who was Bush's Secretary of Commerce from 2005 to 2009.

(Reporting by Bill Rigby; Editing by Jackie Frank and Sandra Maler)